Hemoglobin serves the role of carrying oxygen in blood. Because hemoglobin concentration in blood increases and decreases with vasodilation and vasoconstriction, it is known in the art that vasodilation and vasoconstriction can be detected by measuring the change ΔVhb (t) in hemoglobin concentration.
A simple non-invasive method known in the art for obtaining biological measurements uses a light detection controller and the fact that the change ΔVhb (t) in hemoglobin concentration corresponds to the oxygen metabolism function in a tested subject's body. The change ΔVhb (t) in hemoglobin concentration can be determined by irradiating the tested subject with light whose wavelength varies from visible light to the near-infrared range and measuring the change (received light amount information) in intensity of light that passes through the body.
Hemoglobin bonds with oxygen to become oxyhemoglobin and dissociates from oxygen to become deoxyhemoglobin. It is known that, in the brain, the sites that are activated by blood flow redistribution are supplied with oxygen and that the concentration of oxyhemoglobin created by the bonding with oxygen increases. Hence, the change ΔVoxy in oxyhemoglobin concentration can be measured and used to observe brain activity. Because the absorption spectrum in the visible light to near-infrared spectrum is different between oxyhemoglobin and deoxyhemoglobin, near-infrared light of differing wavelengths (e.g., 780 nm, 805 nm, 830 nm) can be used to determine the change ΔVoxy (t) in oxyhemoglobin concentration and the change ΔVdeoxy (t) in deoxyhemoglobin concentration.
This has led to the development of light measurement devices equipped with light transmission probes and light reception probes that are used for the non-invasive measurement of brain activities. With the light measurement device, near-infrared light is emitted into the brain from the light transmission probes that are positioned on the surface of the tested subject's scalp, and the change in intensity of the near-infrared light that emerges from the brain is detected as the received light amount information by light reception probes that are also positioned on the scalp surface. The near-infrared light passes through the scalp tissue, bone tissue and the like and is absorbed by the oxyhemoglobin or deoxyhemoglobin in the blood. Hence, such light measurement device equipped with light transmission probes and light reception probes and employing near-infrared light of three different wavelengths can be used to obtain from the received light amount information such measurement data as change ΔVoxy (t) in oxyhemoglobin concentration, change ΔVdeoxy (t) in deoxyhemoglobin concentration and, by further calculations based on them, change ΔVhb (t) in hemoglobin concentration over time at the brain's measured sites.
Furthermore, light measurement devices have been developed which measure data such as change ΔVoxy (t) in oxyhemoglobin concentration over time at a plurality of brain measurement sites associated with brain functions such as movement, sensory perception and thinking and are used in the medical field for such purposes as the diagnosis of brain functions and circulatory system disorders. An example of an art that is used in such light measurement devices is near-infrared spectroscopy (“NIRS”). (See, for example, Patent Literature 1.)
With a NIRS, a holder is used so that a plurality of light transmission probes and a plurality of light reception probes are closely attached to the surface of the scalp of the tested subject in a predetermined array. The holder may be shaped like a bowl to fit the shape of the scalp surface. The holder has a plurality of penetrating holes formed therein with a light transmission probe or a light reception probe being inserted into a penetrating hole. This maintains a certain distance (“channel dimension”) between the light transmission probes and the light reception probes and allows received light amount information to be obtained from sites that are located at a specific depth below the scalp surface.
FIG. 2 is a plan view showing one example of the positional relationship among nine light transmission probes and eight light reception probes in a light transmission/reception unit in a NIRS such as the afore-described. The light transmission probes 12 and the light reception probes 13 are positioned so that they alternate with each other in both an oblique direction and a direction orthogonal to the oblique direction.
Even though the light that is emitted from a light transmission probe 12 is received not only by adjacent light reception probes 13 but also by light reception probes 13 that are more distantly located, for the sake of simplicity of the explanation here, it is assumed that the light is detected only by adjacent light reception probes 13. One of the nine light transmission probes 12 is selected and light is emitted from it. This is sequentially repeated so that a total of 24 received light amount information is obtained from channel numbers #1, through #24 shown in FIG. 2.
Measurement data (change ΔVoxy (t) in oxyhemoglobin concentration over time, etc.) that is derived from the received light amount information obtained by NIRS is displayed as images on a monitor screen so that they can be observed by physicians, medical technicians and the like.
One biological measurement method that can be used to diagnose whether or not a biological tissue is normal involves physicians, medical technicians or the like providing a stimulus (referred to as “load” or “task”) to the tested subject and observing the measurement data that is obtained as the brain of the tested subject is activated. FIG. 4 shows one example of a measurement data showing the change ΔVoxy (t) in oxyhemoglobin concentration over time at a particular measurement site (e.g., channel number #1). In the measurement data, the vertical axis plots the change ΔVoxy in oxyhemoglobin concentration in comparison to the initial oxyhemoglobin concentration at the time the NIRS system was worn by the tested subject, and the horizontal axis plots time t.
One example of a biological measurement method such as this involves a physician, medical technician or the like keeping track of time while the tested subject is made to perform the task of moving a finger (finger-tapping) for a set amount of time Y(“task period”). This is followed by a resting period for a set amount of time X(“rest period”) to return to a steady-state. The tested subject is then again made to perform the task of moving the finger for a set amount of time Y which is then followed by resting for a set amount of time X to return to a steady-state. This alternating cycle of the task period followed by the rest period is repeated R times (“task repetition count”). This creates measurement data such as that shown in FIG. 4 depicting the change ΔVoxy (t) in oxyhemoglobin concentration over time that repeats R times.
There are light measurement devices wherein, prior to starting a measurement, a physician, medical technician or the like uses an input screen that is displayed on a monitor screen to input and set measurement conditions such as time Y that defines the task period, time X that defines the rest period and the task repetition count R. FIG. 5 shows one example of an input screen that is used for setting the measurement conditions in a previous light measurement device. A keyboard or the like is used to enter the measurement conditions from the input screen. The measurement conditions may be a task period Y of 10 seconds, rest period X of 40 seconds and a task repetition count R of 5. When the measurement is performed, the light measurement device uses the measurement conditions that are entered to display images that provide instructions to transition from a task period to a rest period when that is required and to transition from a rest period to a task period when that is required. The physician, a medical technician or the like observes the images that are displayed on the monitor screen and issues instructions to the test subject who follows the instructions from the physician, medical technician or the like and alternates between a rest period and a task period for a plurality of times, R.
FIG. 6 shows the theoretical measurement data that should be obtained when a test subject is made to perform the task of moving the finger for a set time Y. The theoretical measurement data is characterized by change over time that increases with a substantially constant slope. However, the actual change ΔVoxy (t) in oxyhemoglobin concentration over time that is generated will be as shown in FIG. 4.
This is because, with a human brain, Mayer Wave variations in arterial pressure occur due to factors such as respiration and blood pressure, regardless of whether the finger moving task is being performed or not. FIG. 7 shows one example of a graph (biological information) showing the change ΔVoxy (t) in oxyhemoglobin concentration over time due to Mayer Waves. As FIG. 7 shows, the period of the Mayer Waves is approximately 10 seconds. However, the Mayer Wave period fluctuates slightly even in the same person and can be, for example, 8 seconds at times and 12 seconds at another.
This means that the measurement data that shows the actual change ΔVoxy (t) in oxyhemoglobin concentration over time such as that shown in FIG. 4 is a superimposition of the theoretical change ΔVoxy (t) in oxyhemoglobin concentration such as that shown in FIG. 6 and the change ΔVoxy (t) in oxyhemoglobin concentration caused by Mayer Wave such as that shown in FIG. 7.
Even though the period of the Mayer Wave is approximately 10 seconds, it is unknown, when the finger movement task is started, as to whether the amount of change ΔVoxy (t) in oxyhemoglobin concentration caused by Mayer Waves is 0.00 mM×cm or 0.10 mM×cm. If the change ΔVoxy (t) in oxyhemoglobin concentration due to Mayer Wave is as shown in FIG. 7, the resulting measurement data becomes as shown in FIG. 4. However, if the Mayer Wave is out of phase by 90° as compared to what is shown in FIG. 7, the resulting measurement data will be as shown in FIG. 9 where the theoretical change ΔVoxy (t) in oxyhemoglobin concentration over time such as that shown in FIG. 6 is superimposed with the change ΔVoxy (t) in oxyhemoglobin concentration over time due to Mayer Wave such as that shown in FIG. 8. Also, if the Mayer Wave is out of phase by 180° as compared to what is shown in FIG. 7, the resulting measurement data will be as shown in FIG. 11 where the theoretical change ΔVoxy (t) inoxyhemoglobin concentration over time such as that shown in FIG. 6 is superimposed with the change ΔVoxy (t) in oxyhemoglobin concentration over time due to Mayer Wave such as that shown in FIG. 10. If the Mayer Wave is out of phase by 270° as compared to what is shown in FIG. 7, the resulting measurement data will be as shown in FIG. 13 where the theoretical change ΔVoxy (t) inoxyhemoglobin concentration over time such as that shown in FIG. 6 is superimposed with the change ΔVoxy (t) in oxyhemoglobin concentration over time due to Mayer Wave such as that shown in FIG. 12.
In other words, when a test subject is made to perform the finger movement task for a set amount of time Y, the measurement data that is obtained may be the measurement data shown in FIG. 4, the measurement data shown in FIG. 9, the measurement data shown in FIG. 11 or the measurement data shown in FIG. 13. The respective measurement data look significantly different from each other as shown in FIG. 14.
It is said that certain mental disorders are characterized by a typical rise (slope) in the change ΔVoxy (t) in oxyhemoglobin concentration. However, it would be difficult for a physician, medical technician or the like to evaluate whether a subject is suffering from depression just by observing measurement data that is affected by variations caused by Mayer Wave. For this reason, with previous biological measurement methods, the tested subject is made to repeat a task for R times during the task period. The resulting R pieces of measurement data that is obtained are added and averaged to reduce the effects of the Mayer Wave, and measurement data that is free of variability is displayed on the monitor screen.